a. Field of the Invention
The invention relates generally to the measurement of instantaneous mass per unit area or thickness of sheet material as it passes a gauging station. More particularly the invention relates to apparatus for measuring mass per unit area or thickness of sheet material taking into account the effect of changes in composition from nominal values as such changes effect the material's transmission coefficient for X-rays and the like.
B. Prior art
In manufacturing such as the rolling of steel, stainless steel, aluminum, copper and brass sheets, and the manufacturing of plastic sheets and the manufacture of paper, it is important to accurately measure the thickness of the material or the mass per unit area as the sheet material is manufactured. If the product is thicker or more massive than desired, the manufacturing material cost is increased. On the other hand, if the material is thinner or less massive than desired the material may be unsuitable for delivery and must be reworked in order to meet delivery specifications or scrapped.
Non-contact X-Ray gauging has been employed on a number of these processes for a number of years, and has been found to be the most satisfactory measurement technique. Other techniques such as contacting mechanical thickness measurement systems, and non-contacting beta-ray gauges have had only limited acceptance for a variety of reasons. Mechanical thickness measurements are undesirable because they may mark the material and because the delicate contacting member tends to "skip" and bounce on high speed sheets and give erroneous readings because of bounce from regions of varying thickness. In addition, mechanical gauges are easily destroyed when the sheet breaks and there is a crash or cobble in a metal rolling mill.
Radioisotope gauges have been employed with moderate success on these processes. However, because of the relatively low intensity of the radiation available, these gauges are not capable of providing a measurement in a short period of time which is sufficiently accurate for control of the machine. Typically, beta gauges require a signal averaging time of 5 seconds or more to achieve measurement accuracies of .+-. 0.5% to .+-. 0.25%. Modern metal rolling mills can be controlled in times of about 0.1 second or less and hence require thickness gauging with an accuracy of .+-. 0.25% or better in times of 0.01 second or less for optimum mill control. Radioisotope gauges have therefore only been useful in where slow speed control of the process is satisfactory, such as paper manufacture.
X-Ray gauges, on the other hand, permit high speed non-contact measurement with wide separation between the product and the measuring equipment. Because of the high intensity available from modern X-Ray equipment, signal averaging times of 0.01 to 0.005 seconds are generally all that is required to provide an accuracy of .+-. 0.25% in the measurement. Nevertheless, because the thickness or mass per unit area is deduced from the absorption of the X-Rays passing through the material and since the X-Ray technique is very sensitive to the composition of the material, large errors in the measurement can and do result from rather minor variations in composition.
For a typical X-Ray gauge the ratio (R.sub.1) of the intensity of the radiation transmitted through the sheet (I.sub.1) to that detected in the absence of the sheet (I.sub.o) may be written: ##EQU1## where m is the mass per unit area of the sheet material, and .mu..sub.1 is the apparent mass absorption coefficient, .rho. is the density, and T is the thickness. Although the absorption of the radiation is due to the mass per unit area of the material in the sheet, each alloy has a known density, which does not vary significantly with minor changes in composition, hence the transmission of radiation may be related directly to thickness. The apparent absorption coefficient results from photoelectric absorption of the X-Rays in the material, and from the coherent and incoherent (Compton) scattering of some of the X-Ray photons out of the angle of acceptance of the detector. Photoelectric absorption is however the predominant effect in these X-Ray thickness gauges. It is clear from equation 1 that precise determination of the true material thickness is reliant upon the absorption coefficient .mu..sub.1 remaining constant and equal to the absorption coefficient determined for the gauge and the particular material composition at the time the gauge is calibrated.
For example many aluminum alloys contain zinc and/or copper concentrations of as much as 5% to 7% with allowable tolerances of about .+-. 0.5% in these elements for a given alloy. If, however, the composition of an aluminum alloy containing zinc shifts slightly say from 4.5% zinc at calibration to 5% zinc in the actual material, a shift in the zinc concentration of 0.5%, the percent error in the thickness measurement may be shown to be about 5% since the absorption coefficient of zinc is approximately 10 to 11 times that of aluminum in the X-Ray energy range of interest (20 to 25 KV). Similar types of errors may occur in steel rolling processes where the concentration of elements such as molybedenum may vary slightly, in copper and brass rolling where the concentration of zinc or lead may vary; and the plastic sheet manufacturing process the concentration of filler materials may vary. In the manufacture of paper substances containing elements such as silicon, calcium, and titanium, which are stronger X-ray absorbers, may be added to the basic paper fiber material to enhance the opacity.
The composition sensitivity of X-Ray thickness gauges outlined above has long been recognized as a primary problem in their use as a measurement tool. Certain U.S. patents teach apparatus for detecting composition variations, as well as use of backscatter radiation for gauging sheet thickness.
U.S. Pat. No. 2,966,587 shows a combination of radiation backscatter and attenuation detectors to determine the percent hydrogen and the hydrogen to carbon ratio of an hydrocarbon.
U.S. Pat. No. 3,188,471 reveals a coin identification apparatus that employs backscatter of beta radiation to determine the atomic number of the coin material, and the X-ray attenuation to determine the weight per unit area.
U.S. Pat. No. 3,499,152 shows a method and apparatus for improving radiation backscatter gauge response that employs a correction signal derived from an attenuation thickness measuring gauge. The fluorescence backscatter gauge is used to measure coating thickness and the attenuation gauge is used to derive a correction signal for variations of in the base thickness.
U.S. Pat. No. 3,569,708 shows a combination backscatter and attenuation gauge inspection apparatus that combines X-ray attenuation and backscatter signals to provide a more accurate indication of the wall thickness of a pipe.
It is an object of the present invention to devise a gauge for measuring weight per unit area of moving sheet material as it passes a gauging station taking into account localized changes in thickness and material composition by means of measuring backscatter and attenuation of an X-ray beam.